The present invention relates to a novel broad band spectrometer.
Thin-film optical waveguides have been successfully integrated with optical fiber technology in the telecommunication industry. U.S. Pat. Nos. 4,664,522 and 4,877,747 describe thin-film waveguides which analyze samples when combined with gratings and detectors. For example, U.S. Pat. No. 4,815,843 to Tiefenthaler and Lukosz describes an optical sensor utilizing a diffraction grating applied onto a substrate which transmits coherent light to a sample through a waveguide and to a detector. The waveguide is simply employed to transmit radiation to the detector. The waveguide was also protected with a blocking overcoat so as not to sense the chemical environment directly. Ultra thin coatings on the grating are reacted with antigen-antibody material to modify the refractive index in the grating region. The changed refractive index due to the thin, modified layer above the grating determines the angle of the incoming coherent light beam that is required for entry into the waveguide. This coupling angle determines the concentration of the chemical species through an intermediate refractive index change.
A notable advance in the analytical instrument field was the thin-film spectroscopic sensor found in U.S. Pat. No. 5,082,629 to Burgess and Goldman. This device made use of thin-film waveguide technology for spectroscopic chemical sensing employing broad band, rather than coherent, light sources. The material to be analyzed contacted the waveguide rather than the ultra-thin artificial reaction layer found in the 4,815,843 patent. In the Burgess and Goldman sensor, the grating region is protected to insure that changes in the chemical environment do not affect the angle that incoming radiation is coupled into and out of the waveguide. A pair of diffraction gratings is etched into the top surface of a dielectric substrate. A waveguide is then deposited over the substrate containing the gratings. "White" light is directed at the first grating through the translucent substrate. At a specific angle of incidence, radiation of one wavelength is coupled into the waveguide. Each angle of radiation entering the waveguide is defined by the properties of the grating, waveguide, substrate, and the sample to be analyzed. Thus, a broad range of angles for the incoming radiation is used to insure that a broad range of wavelengths are coupled into the waveguide. Once in the waveguide, the radiation propagates through the same as if it were passing through an optical fiber. Interaction with the sample occurs via a phenomenon known as attenuated total internal reflection (ATR). Although the radiation beam is contained in the thin-film waveguide, a portion of the beam's electric field extends outside the waveguide and into the sample. Each particular sample extracts power from this extended "evanescent" field. After passing through the waveguide, the remaining radiation enters a detector and results in an absorption spectrum of the sample, once a reference signal is obtained. The absorption spectrum then can be related to the concentration of chemical species in the sample. Such propagation of the radiation beam at each wavelength, after interacting with the sample, is coupled through the waveguide and grating at a unique angle. The dispersion of radiation of different wavelengths is compatible with a photodiode array. The differential illumination of particular elements or diodes in the array are easily transferred into a spectrum of the sample, relating absorbance to wavelength.
Although the Burgess and Goldman device, found in U.S. Pat. No. 5,082,629 is a useful device, it suffered from certain disadvantages. For example, the efficiency of the device is limited since the multi-wavelength radiation must first be brought to an entry grating at a variety of angles to insure that different wavelengths are coupled into the waveguide. Unfortunately, only one wavelength of radiation is coupled into the waveguide through the grating at one angle. Other wavelengths directed at the grating at that particular angle are not coupled into the waveguide and are, thus, not used. Such non-accepted wavelengths add to the stray radiation within the device and impair its performance. As an example, assume that the range of angles needed to couple all visible light (red, green, blue, etc.) into the waveguide encompasses 50.degree. and each degree represents one wavelength. Therefore, only 1/50th of the radiation is used at each angle, ie: at each wavelength. Thus, the device can at best be only 2% efficient at each wavelength. Added to this is the fact that the entry and exit gratings possess an efficiency of about 30%. This means that of the 2% starting radiation at each wavelength, only 9% (30%.times.30%) at most will reach the exit detector. This reduces the overall efficiency to about 0.2% (9%.times.2%). Further reduction in the signal occurs since the inert waveguides composed of materials such as tantalum pentoxide on silica or glass substrates, are typically 50% efficient over a distance of 1 centimeter. Thus the device is realistically only about 0.1% efficient (50%.times.0.2%). Waveguides of longer length have greater propogation losses. In the near-infrared region efficiencies of the Burgess and Goldman device range between 0.2 to 0.6%. Consequently, broad band or "white" light sources are not practically utilized with this device.
Edge couplers combined with optical fibers have been proposed. Edge coupling is not efficient. Single-mode optical fibers having small core diameters of 5-7 microns are 20-30 times thicker than the planar waveguides of the type described in the Burgess and Goldman U.S. Pat. No. 5,082,629. Thus, most of the radiation from such couplers will not enter the waveguide, even under the best alignment conditions. Prism couplers must be placed on the waveguide surface and, thus, destroy the ideal planar geometry of the upper surface of the waveguide in contact with the sample being analyzed. In addition, prism couplers do not allow the placement of the radiation source below or to the side of the substrate. Therefore, prism couplers are exposed to potentially chemical environments. Moreover, efficient coupling of light into a waveguide utilizing a prism requires a variable gap between their surface often, dependant upon the size of ambient dust particles. As a result, reproducibility utilizing prism couplers is difficult to achieve. There also exists a problem with selecting index of refractions between the prisms and the waveguide. Many thin-film waveguides have large refractive indices, e.g. greater than 2.0. Only expensive, higher index prisms such as rutile (TiO.sub.2) could possibly be used in this scenario.
Tapered waveguides are shown in U.S. Pat. No. 4,711,514. Taper couplers have been described by Tien et al in an article entitled "Radiation Fields of a Tapered Film and a Novel Film-to-Fiber Coupler". The taper described is used as a means to couple monochromatic laser light out of a waveguide into an optical fiber using a fine laser beam. A prism was described as being used to couple radiation into the waveguide. Tien et al also specifically indicated that coupling radiation into a taper is much more difficult and would require further development.
Tamir in Chapter 3 of a work entitled "Integrated Optics" indicated that tapered couplers do not appear to be readily applicable in the "leaky-wave theory" that proved to be so useful in dealing prism and grating couplers. The reason given was that the incident surface wave was converted very rapidly to outgoing radiation because the surface-wave mode reaches cut-off conditions in the taper. It was believed that energy was scattered over a wide spectrum of radiation modes, so that a "leaky-wave" cannot be established. Tamir also stated that using a tapered coupler as an input coupler possesses a very small efficiency and is difficult to align and match the form of the incident beam. Tamir also states that due to these disadvantages, "little theoretical efforts have been spent on investigating tapered couplers"
A waveguide-type of spectrometer using broad-band radiation as a source in analyzing chemical samples efficiently would be a great advance in the instrumentation field.